Adding new drive

Hello,

at a DL380G2 with integrated 5i controlller i want to add a hotspare disk to a raid1 array. i use 18,2GB U3 disks. When i will add the new disk i get an error (not enough space on disk)! I can see, that the volume is 2MB smaller (17361MB instead of 17363MB). What's the reason for this problem and how can i get a solution?